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Looking into the cauldron of an exploded star

13 September 2000

Supernovae are one of the most cataclysmic events in the Universe, violent explosions by which stars end their lives. A star may then have a brightness over a billion times that of our Sun and outshine the galaxy in which it lies. Their effects can be observed centuries later. XMM-Newton has been observing the remnants of the Tycho supernova, named after the Danish astronomer Tycho Brahe.

This star in the Cassiopeia constellation first attracted attention in November 1572. At its maximum brightness it reached -4 magnitude and was visible for over 18 months. Tycho Brahe studied it at length but little did he imagine how much astronomers have since learnt about such transient phenomena.

Supernovae occur when a star has exhausted all its nuclear fuel. When the star is massive enough, its core collapses and releases huge amounts of energy, emitting copious amounts of X-rays and blasting the star's envelope into surrounding space. The collapse can also give birth to a black hole or a rapidly rotating neutron star whose radio pulses can also be observed years afterwards.

Supernovae are not uncommon, and statistically can occur about every 30 years in a typical spiral galaxy like our own Milky Way. Others have been observed since Tycho, - Kepler's Star in 1604 and Supernova 1987A, in the neighbouring LMC galaxy, in 1987.

During such explosions, all the chemical elements, except helium and hydrogen are ejected into space. Carbon, oxygen, nitrogen all the elements that make up our bodies, all the rocks of planet Earth. Yet the details of these generation processes, for instance where the elements stem from in the star and how they subsequently mixed, is still unclear.

XMM-Newton, with its great capability for measuring spectra - the "bar-codes" identifying elements - is starting to provide some of the answers. ESA's new X-ray observatory observed the Tycho supernova remnant at the end of June.

The spectra (such as in figure 2) reveal peaks at specific locations, which are signatures of individual chemical elements. The strength of the peak allows astronomers to determine the density of these elements and the temperatures of the surrounding gas. These temperatures can go up to astounding values of millions of degrees, reminiscent of the violence of the initial explosion.

Because XMM-Newton's cameras can clearly discern - or separate out - these different bands, it has been possible to associate chemical elements with portions of the whole view of the supernova remnant. The following four pictures show where the brightest, or most abundant emission from silicon, sulphur, calcium and iron originates.

A detailed examination of what could be described as an "element map" of Tycho shows intriguingly that not all the elements are concentrated in the same spots. This is a clear indication that elements form in different parts of the exploding star and subsequently mix in the turmoil of the expanding gas clouds.

XMM-Newton's uniquely detailed pictures and spectra of Tycho will no doubt keep the astronomers and astrophysicists busy for a long time. And Tycho Brahe would truly be amazed to learn that such a fabulous machine would, 400 years after his first sighting, be delving into the remains of his very bright star

The XMM-Newton observation of the Tycho supernova remnant is currently being analysed by a team of scientists led by Dr. Bernd Aschenbach, Max-Planck Institut fuer Extraterrestrische Physik, Garching b. Muenchen, Germany.